Particle size distribution for fluidized-bed process for making alkylhalosilanes

Abstract: An improved process for the reaction of an alkyl halide with particulate silicon in a fluidized-bed process is described herein. The improvement comprises controlling the particle size of the silicon within a range of one micometer to 85 micrometers. Preferred is when the particle size of the silicon has a mass distribution characterized by a 10th percentile of 2.1 to 6 micrometers, a 50th percentile of 10 to 25 micrometers and a 90th percentile of 30 to 60 micrometers. Most preferred is when the particle size mass distribution of the silicon is characterized by a 10th percentile of 2.5 to 4.5 micrometers, a 50th percentile of 12 to 25 micrometers and a 90th percentile of 35 to 45 micrometers. The process is run in the presence of a catalyst composition comprising copper and other catalysts.

Description: The present invention is an improved process for the reaction of an alkyl halide with particulate silicon in a fluidized-bed process. The improvement comprises controlling the particle size of the silicon within a range of one micrometer to 85 micrometers. The particle size of the silicon has a mass distribution characterized by a 10th percentile of 2.1 to 6 micrometers, a 50th percentile of 10 to 25 micrometers and a 90th percentile of 30 to 60 micrometers. Most preferred is when the particle size mass distribution of the silicon is characterized by a 10th percentile of 2.5 to 4.5 micrometers, a 50th percentile of 12 to 25 micrometers and a 90th percentile of 35 to 45 micrometers. The process is run in the presence of a catalyst composition comprising copper and other catalysts. The present invention relates to an improvement in the Direct Process for producing alkylhalosilanes, wherein an alkyl halide is contacted with particulate silicon in the presence of a copper catalyst. This process was first described by Rochow in U.S. Patents 2,380,996 and 2,380,995. Since then, the process has been refined and modified in numerous ways and is used for producing virtually all commercial alkylhalosilanes in the world today. When one considers that several million pounds of silanes are produced annually and consumed by the silicones commercial effort, it is obvious why even small incremental increases in selectivity and raw material conversion are important to the manufacturers of alkylhalosilanes. Commercially, the largest volume alkylhalosilane manufactured is dimethyldichlorosilane. This alkylhalosilane constitutes the backbone of most high volume commercial silicone products after it has been hydrolyzed and condensed to form silicone polymers. Therefore, it is advantageous to run the Direct Process to maximize the conversion of raw materials to obtain the highest yield of dialkyldihalosilane. Thus, one of the objectives of our invention is to control the Direct Process to maximize the overall yield of alkylhalosilanes, i.e., to cause the process to be as selective as possible in favor of dialkyldihalosilane. A second objective is to maximize the overall yield from the raw materials. The more raw materials that are converted to silanes; the more economical is the process. A third objective is to provide a process where less silicon spent bed of reduced activity in the process is produced. The art has long recognized in the Direct Process that the size of the particulate silicon is important in determining the efficiency of the reaction of alkyl halides with silicon to form alkylhalosilanes. For this invention, the efficiency of the reaction of alkyl halides with silicon is tracked by the amount of silicon charge that is converted to dialkyldihalosilane. However, the prior art has generally taught away from the optimal particle size range defined and claimed by the present inventors. The use of various size ranges of particulate silicon to conduct the Direct Process has been extensively described in the following U.S. Patents: 2,389,931; 2,466,413; 3,133,109; 4,218,387; 4,281,149; 4,307,242; 4,554,370 and Re. 33,452. US-A-2 466 413 discloses the preparation of organohalosilanes by reacting hydrocarbon halides and silicon powder, wherein the distribution, by weight, and the particle size are as follows: 100 % of the particles are smaller than 420 microns in diameter, from 90 to 100 % of the particles are less than 149 microns in diameter and not more than 60 % of the particles are less than 44 microns in diameter, in the presence of a metallic catalyst, especially copper, having a specific particle size and distribution. US-A-3 133 109 discloses the preparation of organohalosilanes by reacting hydrocarbon halides and silicon in the presence of a metallic catalyst, especially copper. The process comprises adding to the fluidized bed make-up silicon having an average particle size greater than that of the silicon particles in the fluidized bed, and comminuting at least a part of the silicon from the fluidized bed, the degree of comminution of said silicon being adjusted to maintain a substantially constant average particle size for the silicon present in the fluidized bed. US-A-4 281 149 discloses a process for treating certain particles of less than 40 microns average diameter size in a silicon reactor where methyl chloride is reacted with silicon particles in the presence of copper to produce chlorosilanes comprising removing particles of less than 40 µm; and abraiding and returning said particles to the reactor. US-A-4 307 242 discloses a process for recovering and recycling elementary silicon from a direct process organohalosilane reactor system, comprising the steps of analyzing a portion of effluent contact mass, such as the silicon fines from a secondary cyclone with a particle size distribution analyzer, determining the fines fractions containing the relatively greatest amounts of impurties and segregating a relatively impure fine fraction from a purer coarse fraction with the aid of an aerodynamic centrifugal classifier. US-A-4 450 282 discloses a process for the preparation of organohalosilanes and especially diorganodihalosilanes by bringing an organohalide in contact with a silicon metal in the presence of a catalyst comprising partially oxidized copper particles having a specific surface area. Quite unexpectedly, we have found that the optimal silicon particle size for conducting the Direct Process in a fluidized-bed reactor is within a range of one micrometer to 85 micrometers and when the particle size of the silicon has a mass distribution characterized by a 10th percentile of 2.1 to 6 micrometers, a 50th percentile of 10 to 25 micrometers and a 90th percentile of 30 to 60 micrometers. Most preferred is when the particle size mass distribution of the silicon is characterized by a 10th percentile of 2.5 to 4.5 micrometers, a 50th percentile of 12 to 25 micrometers and a 90th percentile of 35 to 45 micrometers. Particles outside of our claimed ranges degrade the efficiency of the reactor and it is therefore preferable that sizes outside of our ranges of particles not be added to the reactor. The present invention is a process for the manufacture of alkylhalosilanes described by formula R a H b SiX 4-a-b The process comprises: contacting an alkyl halide described by formula RX with a fluidized-bed of particulate silicon where the particulate silicon has a particle size within a range of one micrometer to 85 micrometers and a particle size mass distribution characterized by a 10th percentile of 2.1 to 6 µm, a 50th percentile of 10 to 25 µm, and a 90th percentile of 30 to 60 µm in the presence of a catalyst composition comprising copper, at a temperature within a range of 250°C. to 350°C.; where each R is independently selected from alkyls comprising one to four carbon atoms; a=0, 1, 2, or 3; b=0, 1, or 2; a + b=1 , 2, or 3; and X is a halogen. The alkylhalosilanes manufactured by the present invention are those described by formula (1). In formula (1), each R is independently selected from alkyls comprising one to four carbon atoms. The substituent R can be methyl, ethyl, propyl, isopropyl, butyl and tert-butyl. In formula (1), the term a can have a value of zero, one, two or three; the term b can have a value of zero, one or two; and the sum of terms a and b can be one, two, or three. The preferred alkylhalosilanes are those having the formula R 2 SiX 2 , where R is methyl or ethyl and X is chlorine. The most preferred alkylhalosilane is dimethyldichlorosilane, i.e. (CH 3 ) 2 SiCl 2 . In the present process, an alkyl halide is contacted with particulate silicon. Alkyl halides useful in our process are described by formula (2), where R and X are as previously described. Preferred is when X is the chlorine atom. The preferred alkyl halide is methyl chloride. Contact of the alkyl halide with the particulate silicon is effected in a fluidized-bed of the particulate silicon. The process can be conducted in standard type reactors for reacting a fluidized-bed of particulates with a gas. The bed can be fluidized using the alkyl halide as the fluidizing media or using a mixture of the alkyl halide with a gas inert in the process as the fluidizing media. The present inventors have discovered that for optimal efficiency of our process, the particulate silicon must have a closely defined particle size or particle size distribution which has not previously been recognized in the art. "Process efficiency" is defined as the percent of silicon converted to dialkyldihalosilane. Process efficiency is a function of the process specificity for the dialkyldihalosilane, amount of silicon converted to monosilanes and of the amount of silicon converted per unit time. The reaction of alkyl halides with particulate silicon is known to be a surface reaction. Alkyl halides react with silicon on catalytically activated silicon surfaces. More available silicon surface gives more potential for reaction in a given volume, so reaction rate is related to the specific surface area of particles available. Smaller particles have high specific surface areas and react away quickly while larger particles have a lower specific surface area and a corresponding lower reaction rate. Furthermore since the silicon particles spend a finite residence time in the reactor, faster reacting small particles are more likely to be consumed to give high silicon conversion and consequently less silicon spent bed. However, we have found that below our particulate silicon size limits the process efficiency is lost. We believe that when the particle size becomes too small or when the particle size distribution contains too large of a mass of particles below the defined ranges, interparticle forces cause the fluidizing gases to "bypass" or "channel" through the fluidized bed. This "bypassing" or channeling" gives poorer solids mixing and heat transfer properties within the fluidized bed. Therefore, we have found that it is desirable to have a silicon particle size for the present process within a range of one micometer to 85 micometers. A preferred silicon particle size is within a range of two to 50 micrometers. It is necessary that the silicon have a particle size mass distribution characterized by a 10th percentile of 2.1 to 6 micrometers, a 50th percentile of 10 to 25 micrometers and a 90th percentile of 30 to 60 micrometers. More preferred is when the particle size mass distribution of the silicon is characterized by a 10th percentile of 2.5 to 4.5 micrometers, a 50th percentile of 12 to 25 micrometers and a 90th percentile of 35 to 45 micrometers. The described distributions are characterized by three percentile sizes. Each percentile describes the particle size in micrometers below which a mass percentage of the size distribution resides: i.e. "10th percentile" - 10 percent of the mass distribution is smaller than the 10th percentile size; "50th percentile" - 50 percent of the mass distribution is smaller than the 50th percentile size; and "90th percentile" - 90 percent of the mass distribution is smaller than the 90th percentile size. The method of making the particle size distributions useful in the present process is not critical. Standard methods for producing particulate silicon can be used. For example, the use of a roller or ball mill is employed to grind silicon lumps. The powdered silicon may be further classified as to particle size distribution by means of screening or mechanical classifiers such as a rotating classifier. The present process is conducted in the presence of a catalyst composition comprising copper. Our process requires the presence of copper as a catalyst within a range of 0.1 to 10 weight percent of the silicon present in the process. The source of the copper added to the process may be powdered copper metal, powdered silicon-copper alloy, a compound of copper or a mixture of two or more sources of copper. The copper compound may be, for example, cuprous chloride. In addition to copper, the catalyst composition may employ other metals as catalysts. The scope of other metals contemplated as catalysts by us are those metals known to those skilled in the art as promoters of the Direct Process. Examples of such catalytic metals are described in U.S. Patents 4,602,101; 4,946,978; 4,762,940 and Re. 33,452. These catalytic metals include, for example, phosphorous, phosphorous compounds, zinc, zinc compounds, tin, tin compounds and mixtures thereof. A preferred catalyst composition for our process comprises (on an elemental basis by weight): 0.1 to 10 weight percent copper based on silicon present in the process, 50 to 10,000 ppm zinc, 5 to 200 ppm tin and 25 to 2,500 ppm phosphorous. The process can be conducted at a temperature within a range of 250°C. to 350°C. The preferred temperature for conducting the present process is within a range of 270°C. to 320°C. The following examples are offered to illustrate the present invention. Example 1 (Not within the scope of the present invention) A control baseline for the reaction of metallurgical grade silicon with methyl chloride was established. A mixture comprising 100 parts of refined, ground metallurgical grade silicon (aluminum = 0.16 Wt.%, iron = 0.32 Wt.% and calcium = 0.004 Wt.%), 6.5 parts of cuprous chloride, 600 ppm brass (50 Wt.% zinc), 36 ppm tin, and 2000 ppm copper phosphorus alloy, was formed. The particle size mass distribution of the silicon was characterized as a 10th percentile of 2.8 µm, a 50th percentile of 22 µm and a 90th percentile of 81 µm. The mixture was charged to a reactor similar to that described in U.S. Patent No. 4,218,387. The temperature of the reactor was maintained at about 315°C. by means of a constant temperature bath. The reactor was purged for 15 minutes with nitrogen gas. The nitrogen purge was then shut off and methyl chloride gas was fed to the reactor for a total of 44 hours during which time all products and unreacted methyl chloride were collected in a cold-trap. Weight loss of the reactor was used as an indicator of silicon conversion. The liquid collected in the cold-trap was analyzed by gas chromotography (GC) using a thermal conductivity (TC) detector. The performance of the silicon in each run was calculated as the weight fraction of silicon consumed times the weight percent of dimethyldichlorosilane as a percent of the total weight of silane products formed. The average silicon performance for this series of runs was determined to be 77.9 percent. Example 2 Samples of particulate metallurgical grade silicon having various particle size mass distributions were evaluated in a process similar to that described in Example 1. The specific particle size mass distribution and silicon performance is provided in Table 1. Each sample of a specific particle size mass distribution was run in duplicate and the average silicon performance for the two runs is also reported in Table 1 under the heading "Si-Perf." The 10th, 50th and 90th percentile particle mass distributions were determined by use of a Micromeritics Sedigraph Model 5000. The sieve mesh size that the silicon powder passed is also indicated in Table 1. Results were analyzed and silicon performance calculated as described in Example 1. Silicon Particle Size Distribution Effects on Performance Sample No. Particle Mass Distribution (µm) 10th 50th 90th Mesh Si-Perf. 1 2.2 13.0 36.5 <325 81.9 2 1.6 10.7 35.0 <325 67.8 3 1.7 10.2 31.0 <325 69.1 4 2.4 15.7 49.0 <230 80.4 5 2.2 14.0 40.5 <270 84.2 6 2.4 13.5 41.1 <325 83.1 7 2.0 11.2 32.3 <400 75.8

Claims: A process for the manufacture of alkylhalosilanes described by formula R a H b SiX 4-a-b , the process comprising: contacting an alkyl halide described by formula RX with a fluidized-bed of particulate silicon where the particle silicon has a size within a range of one micometer to 85 micrometers and a particle size mass distribution characterized by a 10th percentile of 2.1 to 6 µm, a 50th percentile of 10 to 25 µm, and a 90th percentile of 30 to 60 µm in the presence of a catalyst composition comprising copper, at a temperature within a range of 250°C. to 350°C; where each R is independently selected from alkyls comprising one to four carbon atoms; a=1, 2, or 3; b=0, 1 or 2; a + b=1 , 2, or 3; and X is a halogen. A process according to claim 1 where the particulate silicon has a particle size mass distribution characterized by a 10th percentile of 2.5 to 4.5 micrometers, a 50th percentile of 12 to 25 micrometers and a 90th percentile of 35 to 45 micrometers. A process according to claim 1 where the alkylhalosilane is dimethyldichlorosilane. A process according to claims 1, 2 or 3 where the alkyl halide is methyl chloride; the catalyst composition comprises on an elemental basis, by weight, 0.1 to 10 weight percent copper based on silicon present in the process, 50 to 10,000 ppm zinc, 5 to 200 ppm tin and 25 to 2,500 ppm phosphorous; and the temperature is within a range of 270°C. to 320°C.

Inventors: FREEBURNE STEVEN KERRY, HALM ROLAND LEE, KOHANE JOSEPH PETER, WINELAND JONATHAN DAVID, FREEBURNE, STEVEN KERRY, HALM, ROLAND LEE, KOHANE, JOSEPH PETER, WINELAND, JONATHAN DAVID
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1994
Main classification: C07F7
IPC codes: C07B61, C07F7, B01J27